Switch everything to platforms

This patch switches the codebase over from using the "cpu"
mechanism to using bazel platforms. See
https://docs.bazel.build/versions/master/platforms.html for some more
information.

Most of the substantial changes are in //tools. Instead of using
`cc_toolchain_suite` rules, we now use regular `toolchain` rules that
are registered in the WORKSPACE. That also means that bazel now uses
the target platform to select the compiler.

All --cpu=* arguments should now be --config=* arguments. For example,
`--cpu=roborio` should now be `--config=roborio`. The CI script and
all documentation has been updated to reflect that.

The remainder of the changes revolve around tagging all targets with
`target_compatible_with`.  The old mechanism allowed us to specify
repo-wide defaults. The new mechanism does not. That means every
target that didn't have any compatibility specified, now requires
compatibility with `@platforms//os:linux`.

I used buildozer for the vast majority of `target_compatible_with`
changes. buildozer automatically buildifies any BUILD files it
touches. That means this patch also contains a few non-functional
changes that I was too lazy to remove.

Change-Id: I66d6e6ad9161520ee397597cdb492585820a3acd
diff --git a/y2014_bot3/control_loops/rollers/BUILD b/y2014_bot3/control_loops/rollers/BUILD
index e0d7dc6..2f9d7c4 100644
--- a/y2014_bot3/control_loops/rollers/BUILD
+++ b/y2014_bot3/control_loops/rollers/BUILD
@@ -8,6 +8,7 @@
         "rollers_goal.fbs",
     ],
     gen_reflections = 1,
+    target_compatible_with = ["@platforms//os:linux"],
 )
 
 flatbuffer_cc_library(
@@ -16,6 +17,7 @@
         "rollers_position.fbs",
     ],
     gen_reflections = 1,
+    target_compatible_with = ["@platforms//os:linux"],
 )
 
 flatbuffer_cc_library(
@@ -24,6 +26,7 @@
         "rollers_output.fbs",
     ],
     gen_reflections = 1,
+    target_compatible_with = ["@platforms//os:linux"],
 )
 
 flatbuffer_cc_library(
@@ -32,6 +35,7 @@
         "rollers_status.fbs",
     ],
     gen_reflections = 1,
+    target_compatible_with = ["@platforms//os:linux"],
 )
 
 cc_library(
@@ -42,6 +46,7 @@
     hdrs = [
         "rollers.h",
     ],
+    target_compatible_with = ["@platforms//os:linux"],
     deps = [
         ":rollers_goal_fbs",
         ":rollers_output_fbs",
@@ -57,6 +62,7 @@
     srcs = [
         "rollers_main.cc",
     ],
+    target_compatible_with = ["@platforms//os:linux"],
     deps = [
         ":rollers_lib",
         "//aos:init",